Ultra High Molecular Weight Polyethylene Articles And Methods Of Forming Ultra High Molecular Weight Polyethylene Articles

ABSTRACT

The present invention generally provides implantable articles and methods of forming implantable articles from a crosslinked ultrahigh molecular weight polyethylene (“UHMWPE”) blend stabilized with Vitamin E. The crosslinked UHMWPE blend may be prepared by combining the UHMWPE material and vitamin E prior to irradiating the UHMWPE blend with electron beam radiation at a sufficient radiation dose rate to induce crosslinking. The crosslinked UHMWPE blend may be incorporated into a variety of implants, and in particular, into endoprosthetic joint replacements

CROSS REFERENCE TO RELATED ED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/465,743, filed Aug. 18, 2006, which is a continuation of PCT PatentApplication No. PCT/EP2005/008967, filed Aug. 18, 2005, both of whichare incorporated herein by reference in their entireties as ifcompletely set forth herein below.

BACKGROUND

Many endoprosthetic joint replacements currently implanted in patientsinclude a highly polished metal or ceramic component articulating on anultra high molecular weight polyethylene (UHMWPE) material or blend.Wear and abrasion resistance, coefficient of friction, impact strength,toughness, density, biocompatibility and biostability are some of theproperties that make UHMWPE a suitable material for such implants.Although UHMWPE has been used in implants for many years, there iscontinuing interest in the wear and durability characteristics ofimplants incorporating UHMWPE.

One method employed to improve the durability and other physicalcharacteristics of UHMWPE implants has been to expose such implants toradiation, for example gamma radiation or electron beam radiation, toinduce crosslinking in the UHMWPE. Similar radiation sources have alsobeen used to sterilize UHMWPE implants prior to distribution.

Despite the benefits of irradiating UHMWPE implants, the irradiationprocess may lead to increased rates of oxidation in the UHMWPE implant.In particular, irradiation has been shown to generate free radicals,which react in the presence of oxygen to form peroxyl radicals. Thesefree radicals and peroxyl radicals may react with the polyethylenebackbone and with each other to form oxidative degradation products andadditional radical species. This cycle of oxidation product and radicalspecies formation may occur over several years (both prior to and afterimplantation) as oxidation levels in the implant increase.

One method that has been utilized to reduce oxidation in irradiatedUHMWPE materials is the addition of a stabilizing component to theUHMWPE material to inhibit the oxidation cycle. However, the addition ofa stabilizer or stabilizing components, such as vitamin E, to UHMWPEprior to irradiation has been shown to have an adverse effect oncrosslinking during irradiation. See Parth et al., “Studies on theeffect of electron beam radiation on the molecular structure ofultra-high molecular weight polyethylene under the influence ofα-tocopherol with respect to its application in medical implants,”Journal of Materials Science Materials In Medicine, 13 (2002), pgs.917-921.

For this reason, the addition of stabilizers to UHMWPE materials afterforming and irradiating via diffusion has been proposed. See e.g., PCTPublished Application No. WO 2004/101009. However, the addition ofstabilizers after irradiation has several limitations. For example,vitamin E diffusion may provide a less uniform distribution ofstabilizer in UHMWPE than pre-irradiation mixing. Diffusion of thevitamin E may also require separate irradiation steps to inducecrosslinking prior to adding vitamin E and then to sterilize the implantafter adding vitamin E.

Therefore, it would be beneficial to provide a method of forming acrosslinked UHMWPE material for use in implanted articles that overcomesone or more of these limitations.

SUMMARY

In one embodiment, the present invention provides an implantable articleformed from a crosslinked ultrahigh molecular weight polyethylene(“UHMWPE”) blend. The crosslinked UHMWPE blend may be prepared bycombining a UHMWPE material with a stabilizer, such as vitamin E, andother optional additives reported herein to form a UHMWPE blend, andthen by irradiating the UHMWPE blend with a suitable radiation source,such as electron beam radiation, at a sufficient radiation dose rate toinduce crosslinking. The resulting crosslinked UHMWPE blend may have aswell ratio of less than about 4, and at least about 0.02 w/w % vitaminE is uniformly dispersed within at least a surface region of an articleformed from the blend. According to this invention, the vitamin E may beuniformly distributed from the surface of the article to a depth of atleast about 5 mm. The crosslinked UHMWPE blend of the present inventionmay be incorporated into a variety of implants, and in particular, intoendoprosthetic joint replacements

BRIEF DESCRIPTION OF FIGURES

FIGS. 1A-1C are flow-charts illustrating methods of preparing UHMWPEimplants according to embodiments of the present invention.

FIGS. 2A-2B are flow-charts illustrating methods of preparing UHMWPEimplants according to additional embodiments of the present invention.

FIG. 3 is a line graph illustrating the swell ratio of several UHMWPEsamples, described in the Example, at various radiation dose rates.

FIGS. 4A-4C are bar graphs illustrating the TVI (4A), swell ratio (4B)and soluble fraction (4C), of several UHMWPE samples.

FIG. 5 is a line graph illustrating the vitamin E concentration ofseveral UHMWPE samples at a range of depths.

FIG. 6 is a prior art line graph showing the vitamin E index of samplesprepared pursuant to U.S. Published Application No. 2004/0156879.

FIG. 7 is a line graph showing the oxidation levels of several UHMWPEsamples at a range of depths.

FIG. 8 is a bar graph showing the tensile strength of several UHMWPEsamples.

FIG. 9 is a bar graph showing the elongation percent at break of severalUHMWPE samples.

FIG. 10 is a bar graph showing the Charpy impact strength of severalUHMWPE samples.

DETAILED DESCRIPTION

UHMWPE is a semicrystalline, linear homopolymer of ethylene, which maybe produced by stereospecific polymerization with a Ziegler-Nattacatalyst at low pressure (6-8 bar) and low temperature (66-80° C.). Thesynthesis of nascent UHMWPE results in a fine granular powder. Themolecular weight and its distribution can be controlled by processparameters such as temperature, time and pressure. UHMWPE generally hasa molecular weight of at least about 2,000,000 g/mol.

Suitable UHMWPE materials for use as raw materials in the presentinvention may be in the form of a powder or mixture of powders. TheUHMWPE material may be prepared almost entirely from UHMWPE powder, ormay be formed by combining UHMWPE powder with other suitable polymermaterials. In one embodiment, the UHMWPE material may include at leastabout 50 w/w % UHMWPE. Examples of suitable UHMWPE materials include GUR1020 and GUR 1050 available from Ticona Engineering Polymers. Suitablepolymer materials for use in combination with the UHMWPE materials mayinclude disentangled polyethylene, high pressure crystallizedpolyethylene and various other “super tough” polyethylene derivatives.In addition, biocompatible non-polyethylene polymers may also besuitable for use in certain embodiments.

Suitable additives to the UHMWPE material include radiopaque materials,antimicrobial materials such as silver ions, antibiotics, andmicroparticles and/or nanoparticles serving various functions.Preservatives, colorants and other conventional additives may also beused.

Suitable stabilizers for addition to the UHMWPE material generallyinclude materials that can be added in an effective amount to the UHMWPEmaterial in order to, at least in part, inhibit the oxidation cyclecaused by irradiation of UHMWPE. Vitamin E is particularly suitable foruse in embodiments of the present invention. As used herein “vitamin E”refers generally to derivatives of tocopherol including α-tocopherol.Other suitable stabilizers may include phenolic antioxidants such asbutylated hydroxytoluene, and ascorbic acid.

The vitamin E stabilizer and UHMWPE material may be combined via anumber of known processes to form a UHMWPE blend. Such processes includephysical mixing, mixing with the aid of a solvent, mixing with the aidof a solvent (e.g. Co2) under supercritical temperature and pressureconditions, and ultrasonic mixing. Suitable mixing processes of thesetypes are also described, for example, in U.S. Pat. Nos. 6,448,315 and6,277,390, the disclosures of which are hereby incorporated byreference. In one embodiment, vitamin E is dissolved in ethanol and isdrop-wise added to a powdered UHMWPE material while mixing. The ethanolmay then be removed via a vacuum dryer or similar apparatus.

FIGS. 1A-1C and 2A-2B are flowcharts illustrating methods for preparingimplants from UHMWPE blends according to embodiments of the presentinvention. The general steps for processing the implant include aconsolidating/compressing the UHMWPE blend, crosslinking the UHMWPEblend, manufacturing an implant from the compressed UHMWPE blend,packaging the implant, and sterilizing the packaged implant. Asreflected in FIGS. 1 AC and 2A-2B, these steps may be carried out invarying order, in multiple steps, or simultaneously in accordance withembodiments of the present invention.

The UHMWPE blend may first be consolidated and/or compressed intosuitable form for use as (or as part of) a prosthetic device or otherimplant. Suitable compression and/or consolidation techniques include,for example, compression molding, direct compression molding, hotisostatic pressing, ram extrusion, high pressure crystallization,injection molding, sintering or other conventional methods ofcompressing and/or consolidating UHMWPE. If desired, thecompressed/consolidated UHMWPE blend may be further processed ormanufactured by milling, machining, drilling, cutting, assembling withother components, and/or other manufacturing or pre-manufacturing stepsconventionally employed to manufacture implants from UHMWPE.

Prior to and/or after processing the implant as reported above, theUHMWPE blend may be crosslinked by exposure to radiation at a highradiation dose and/or dose rate to form a crosslinked UHMWPE blend. Inone embodiment, the UHMWPE blend may be exposed to electron beamradiation at a dose of at least about 25 kiloGrey, more particularly atleast about 80 kiloGrey, and even more particularly at least about 95kiloGrey. In another embodiment, the UHMWPE blend may be exposed toradiation at a dose rate of at least 1 MegaGrey per hour, moreparticularly at least about 15 MegaGrey per hour, and even moreparticularly about 18 MegaGrey per hour. In certain embodiments, thedesired radiation dose may be achieved in a single exposure step at ahigh dose rate. In other embodiments, a series of high dose rateirradiation steps may be employed to expose the UHMWPE blend to adesired dose of radiation.

In certain embodiments, the radiation source is electron beam radiation.Electron beam radiation exposure may be performed using conventionallyavailable electron beam accelerators. One commercial source for such anaccelerator is IBA Technologies Group, Belgium. Suitable acceleratorsmay produce an electron beam energy between about 2 and about 50 MeV,more particularly about 10 MeV, and are generally capable ofaccomplishing one or more of the radiation doses and/or dosage ratesreported herein. Electron beam exposure may be carried out in agenerally inert atmosphere, including for example, an argon, nitrogen,vacuum, or oxygen scavenger atmosphere. Exposure may also be carried outin air under ambient conditions according to one embodiment. Gamma andx-ray radiation may also be suitable for use in alternate embodiments ofthe invention. The present invention need is not necessarily limited toa specific type of source of radiation.

Optionally, prior to and/or after electron beam irradiation, the UHMWPEblend may be subjected to one or more temperature treatments. In oneembodiment, the UHMWPE blend may be heated above room temperature, moreparticularly above about 100° C., even more particularly between about120° C. and 130° C., prior to irradiation. U.S. Pat. No. 6,641,617 toMerril et al., which is hereby incorporated by reference, reportsmethods of employing such temperature treatment steps in greater detail.In another embodiment, the UHMWPE blend may remain at room temperatureor may even be cooled below room temperature, for example, below theglass transition temperature of the UHMWPE blend. After irradiation, thecrosslinked UHMWPE blend may be annealed at a temperature of up to about200° C. for up to about 72 hours, more particularly at about 150° C. forabout 5 hours. Alternatively or additionally, the crosslinked UHMWPEblend may be subjected to the mechanical annealing processes reported inU.S. Pat. No. 6,853,772 to Muratoglu, which is hereby incorporated byreference. In one embodiment, however, no pre- or post-irradiationtemperature and/or annealing treatments are performed.

As part of the implant manufacturing process, additional components maybe combined with the UHMWPE blend at any time during the processreported herein. In one embodiment, tribological components such asmetal and/or ceramic articulating components and/or preassembled bipolarcomponents may be joined with the UHMWPE blend. In other embodiments,metal backing (e.g. plates or shields) may be added. In furtherembodiments, surface components such a trabecular metal, fiber metal,beats, Sulmesh® coating, meshes, cancellous titanium, and/or metal orpolymer coatings may be added to or joined with the UHMWPE blend. Stillfurther, radiomarkers or radiopacifiers such as tantalum, steel and/ortitanium balls, wires, bolts or pegs may be added. Further yet, lockingfeatures such as rings, bolts, pegs, snaps and/or cements/adhesives maybe added. These additional components may be used to form sandwichimplant designs, radiomarked implants, metal-backed implants to preventdirect bone contact, functional growth surfaces, and/or implants withlocking features.

A variety of implants, and in particular endoprosthetic jointreplacements, may be prepared by employing the methods reported herein.Examples of such implants include artificial hips and knees, cups orliners for artificial hips and knees, spinal replacement disks,artificial shoulder, elbow, feet, ankle and finger joints, mandibles,and bearings of artificial hearts.

After manufacturing of the implant has been completed, it may bepackaged and sterilized prior to distribution. Packaging is generallycarried out using either gas permeable packaging or barrier packagingutilizing a reduced oxygen atmosphere. Because the presence of vitamin Ein the UHMWPE blend inhibits the oxidation cycle, conventional gaspermeable packing may be suitable for embodiments of the presentinvention. Barrier packaging with an inert gas backfill (e.g. argon,nitrogen, oxygen scavenger) is also suitable.

As reflected in FIGS. 1A-1C and 2A-2B, sterilization may be accomplishedeither by radiation exposure during crosslinking of the UHMWPE blend, oras part of a separate processing step. A number of conventionalsterilization techniques exist including gas plasma sterilization,ethylene oxide sterilization, gamma radiation sterilization and e-beamradiation. In the embodiments illustrated in FIGS. IA, 1C and 2B,crosslinking is carried out prior to packaging. In the embodimentsillustrated in FIGS. 1B and 2A, sterilization and crosslinking arecarried out by e-beam irradiation in a single step after packaging theimplant.

Sterilization generally occurs after packaging. In certain embodiments,sterilization is carried out at the same time as crosslinking, andtherefore utilizes e-beam radiation. In embodiments in whichcrosslinking occurs before sterilization, additional suitablesterilization methods include gamma irradiation (either inert or inair), gas plasma exposure or ethylene oxide exposure.

As further exemplified in the Examples set forth below, the crosslinkedUHMWPE blends produced according to embodiments of the present inventionmay have several beneficial characteristics. Notably, such blendsexhibit lower levels of oxidation when compared to unstabilized UHMWPEmaterials, while still exhibiting suitable levels of crosslinking. Theuse of a high radiation dose rate or a series of high radiation doserates, at least in part, contributes to improved crosslinking densitiesfor the UHMWPE blend, which is contrary to prior art reports thatsuggest that suitable crosslinking densities are difficult to achievewhen irradiating stabilized UHMWPE blends.

Also, such UHMWPE blends may have a generally uniform distribution ofvitamin E at least a surface region of the blend. As used herein, thephrase “surface region” refers to a region of a crosslinked UHMWPE blendextending from a surface of the blend to some depth or range of depthsbelow the surface. For example, the implants formed from the crosslinkedUHMWPE blend of certain embodiments may exhibit a substantially uniformdistribution of vitamin E to a surface depth of at least 3 mm, moreparticularly, at least 5 mm. Other embodiments may exhibit asubstantially uniform distribution of vitamin E to a surface depth of atleast 10 mm, more particularly at least 15 mm, even more particularly atleast 20 mm. In further embodiments, the UHMWPE blend may have asubstantially uniform distribution of vitamin E throughout the blend.

EXAMPLES

Table 1 sets forth the processing parameters for Samples A-I.

TABLE 1 Vitamin Preheating Raw E- Before Irradiation Material ContentIrradiation Dose Irradiation Irradiation Radiation Annealing SAMPLE GURw/w % ° C. kGy Dose Rate Environment Source Process A 1020 0.0 N/A 25-400.5 to 10 N₂ Gamma N/A (kGy/h) B 1020 0.0 N/A N/A N/A N/A N/A N/A C 10200.1 N/A N/A N/A N/A N/A N/A D 1020 0.1 N/A 25-40 0.5 to 10 N, Gamma N/AE 1020 0.0 120 95 18 Air eBeam 150° C., (MGy/h) 5 h F 1020 0.1 120 95 18Air eBeam 50° C., (MGy/h) 5 h G 1020 0.1 N/A 95 18 Air eBeam N/A (MGy/h)H 1020 0.1 N/A 95 0.5 to 10 Air Gamma N/A (kGy/h) I 1050 0.0 120 95 18Air eBeam 150° C. (MGy/h) 5 h

As set forth in Table 1, GUR 1020 and GUR 1050 brand UHMWPE powders areavailable from Ticona GmbH, FrankfurtMain, DE. The vitamin E used forSamples C, D and F-H was α-tocopherol obtained from DSM NutritionalProducts AG, Basel, Switzerland.

For Samples C, D and F-H, the α-tocopherol was dissolved in ethanol in aconcentration of 50 g/l and mixed into the UHMWPE drop-wise using aNauta-Vrieco brand screw-cone mixer. The ethanol was then removed fromthe UHMWPE blend in a vacuum dryer at 50° C. for 6 hours, resulting in aUHMWPE blend having a concentration of α-tocopherol of about 0.1 w/w %,The resulting UHMWPE blend was then sintered for 7 hours at 220° C. and35 bar to produce UHMWPE plates having a thickness of 60 mm and adiameter of 600 mm. Homogeneity of the α-tocopherol in the UHMWPE blendwas measured by standard HPLC methods and determined to vary up to +/−2%from the desired content.

Samples A, D and H were irradiated using a Studer IR-168 GammaIrradiator utilizing a Co⁶⁰ radiation source. Samples E-G and I wereirradiated using a 10 MeV Rhodotron electron accelerator available fromIBA SA, Louvain-La-Neuve using a 120 kW power setting.

RESULTS

FIG. 3 shows a line graph illustrating the swell ratio of unstabilizedpolyethylene versus a polyethylene blend stabilized with Vitamin E. Theswell ratio is a useful indicator of the crosslinking density of aparticular material. In particular, lower relative swell ratios areindicative of higher levels of crosslinking, and vice versa. The swellratio was determined according to ASTM F2214-02. Specifically, 4-6 mmcubes of each of Samples H, F, G and E were placed in a container filledwith o-Xylene at 25° C. and placed in a dynamical mechanical analyzer(DMA, DMA 7 e available from Perkin Elmer) for 10 minutes. A firstsample height (Ho) was then taken for each sample. The samples were thenheated at a rate of 5K/min to a maintained temperature of 130° C. Asecond sample height (Hf) was then taken after 120 minutes at 130° C.The swell ratio was then calculated according to the following equation:

q _(s)=(Ht/Ho)³

The data points for the lower flat line include a swell ratio standardfor unstabilized UHMWPE (obtained from the interlaboratory comparison inASTM F2214-02 at a dose rate of 89 kGy) and unstabilized Sample E. Thesedata points indicate that dose rates do not have a substantial effect oncrosslink density. The data points for the upper descending line includeSamples H, F and G. Notably, the increased irradiation dosage rates usedfor Samples F and G resulted in a decreased swell ratio when compared tosample H, and consequently, an increased crosslink density.

FIGS. 4A-4C are three bar graphs illustrating several characteristics ofSamples 13, C, and E-H. FIG. 4A is a bar graph illustrating thetrans-Vinylene Index (TVI) levels of the Samples. The TVI was determinedby the method described in Muratoglu et al., “Identification andquantification of irradiation in UHMWPE through trans-vinylene yield.”TVI levels are an indicator of the radiation absorption efficiency ofUHMWPE. FIG. 4A indicates that the Samples E and F, which were preheatedbefore irradiation and annealed after irradiation, possessed higherradiation absorption efficiency than other samples.

FIG. 4B is a bar graph illustrating the swell ratio of the same samplesreported in FIG. 4A. Notably, Sample H, which was gamma-irradiated,shows a higher swell ratio (and therefore lower crosslink density) thanthe e-beam irradiated Samples E, F and G.

FIG. 4C is a bar graph of the soluble fraction of the samples reportedin FIG. 4A. The soluble fraction indicates the percentage of fullycrosslinked material in the sample. The soluble fraction for each samplewas determined in accordance with ASTM 2765-01. Specifically, powderedUHMWPE was taken from a location 10 mm under the surface of the sampleby a rasping technique. This sample was then weighed in a wire mesh andbackfluxed for 12 hours in xylene. After backfluxing, the remaining gelportion was placed in a vacuum furnace and dried at a temperature of140° C. and a pressure of less than 200 mbar, and was then conditionedin an exsiccator before being weighted again. The resulting gel portionand soluble portion was computed by weighing the sample before and afterthe procedure. Sample H, which was gamma irradiated, shows a highersoluble fraction than e-beam irradiated Samples E, F and G.

FIG. 5 is a line graph indicating the vitamin E content at a range ofdepths from the surface of Samples C, F, G and H. FIG. 5 indicates thata uniform vitamin E concentration is maintained in each Sample in asurface region at least up to the measured depth of 20 mm. This uniformdistribution of vitamin E is particularly notable when compared to PriorArt FIG. 6 reported in U.S. Published Application No 2004/0156879, inwhich the vitamin E index of diffused vitamin E samples steadilydecreased as depth increased.

FIG. 7 is a line graph illustrating the oxidation levels of Samples E,F, G and H. Notably, oxidation levels at certain depths from the surfaceof the sample material were higher for Sample E (did not include vitaminE) and Sample H (gamma irradiated) as compared to Samples G and F(e-beam irradiated).

FIGS. 8-10 are a series of bar graphs illustrating various mechanicalproperties of Samples A, D, E, F, G, H and I. FIG. 8 illustrates themechanical strength of each sample, and generally indicates that thepre-heating and annealing processing methods utilized with samples E, Fand I resulted in somewhat decreased mechanical strength as compared tothe cold irradiation method used for Sample G and H. FIG. 9 illustratesthe elongation percent at the breaking point of each sample. FIG. 10illustrates the impact strength of each sample based on the Charpyimpact scale (kJ/m²), and generally indicates that the presence ofvitamin E increases the impact strength of crosslinked UHMWPE.

1. A method of forming a crosslinked ultrahigh molecular weightpolyethylene blend comprising: combining an amount of vitamin E andultrahigh molecular weight polyethylene to form a ultrahigh molecularweight polyethylene blend; and irradiating the ultrahigh molecularweight polyethylene blend with electron beam radiation at an absorbeddose of at least 60 kiloGrey and a dose rate of at least 1 MegaGrey perhour to crosslink said ultrahigh molecular weight polyethylene blend. 2.The method of claim 1 wherein the amount of vitamin E combined with theblend is between about 0.02 w/w % and about 2.0 w/w %.
 3. The method ofclaim 1 wherein the amount of vitamin E combined with the blend isbetween about 0.05 w/w % and about 0.4 w/w %.
 4. The method of claim 1wherein at least some of the vitamin E is uniformly dispersed within asurface region of the blend and the surface region extends from anexposed surface of the blend to a depth of at least about 5 millimetersfrom the surface of the blend.
 5. The method of claim 4 wherein thesurface region extends from an exposed surface of the blend to a depthof at least about 15 millimeters from the surface of the blend.
 6. Themethod of claim 4 wherein at least about 0.02 percent by weight of thevitamin E is uniformly dispersed within the surface region of the blend.7. The method of claim 5 wherein at least about 0.04 percent by weightof the vitamin E is uniformly dispersed within the surface region of theblend.
 8. The method of claim 1 wherein the vitamin E is dispersedsubstantially uniformly throughout the blend.
 9. The method of claim 1further including preheating the ultrahigh molecular weight polyethyleneblend to above about room temperature prior to irradiation.
 10. Themethod of claim 1 further including preheating the ultrahigh molecularweight polyethylene blend to between about 120° C. to and about 130° C.prior to irradiation.
 11. The method of claim 1 wherein the ultrahighmolecular weight polyethylene blend is annealed after irradiation at atemperature up to about 200° C.
 12. An implantable article comprising: acrosslinked ultrahigh molecular weight polyethylene blend wherein theblend is produced by: combining an amount of vitamin E and ultrahighmolecular weight polyethylene to form a ultrahigh molecular weightpolyethylene blend; and irradiating the ultrahigh molecular weightpolyethylene blend with electron beam radiation at an absorbed dose ofat least 60 kiloGrey and a dose rate of at least 1 MegaGrey per hour tocrosslink said ultrahigh molecular weight polyethylene blend.
 13. Theimplantable article of claim 12 wherein the amount of vitamin E combinedwith the blend is between about 0.02 w/w % and about 2.0 w/w %.
 14. Theimplantable article of claim 12 wherein the amount of vitamin E combinedwith the blend is between about 0.05 and about 0.4 w/w %.
 15. Theimplantable article of claim 12 wherein at least some of the vitamin Eis uniformly dispersed within a surface region of the blend and thesurface region extends from an exposed surface of the blend to a depthof at least about 5 millimeters from the surface of the blend.
 16. Theimplantable article of claim 15 wherein the surface region extends froman exposed surface of the blend to a depth of at least about 15millimeters from the surface of the blend.
 17. The implantable articleof claim 15 wherein at least about 0.02 percent by weight of the vitaminE is uniformly dispersed within the surface region of the blend.
 18. Theimplantable article of claim 15 wherein at least about 0.04 percent byweight of the vitamin E is uniformly dispersed within the surface regionof the blend.
 19. The implantable article of claim 12 wherein thevitamin E is dispersed substantially uniformly throughout the blend. 20.The implantable article of claim 12 wherein the ultrahigh molecularweight polyethylene blend is preheated prior to irradiation to aboveabout room temperature.
 21. The implantable article of claim 12 whereinthe ultrahigh molecular weight polyethylene blend is preheated prior toirradiation to between about 120° C. to and about 130° C.
 22. Theimplantable article of claim 12 wherein the ultrahigh molecular weightpolyethylene blend is annealed after irradiation at a temperature up toabout 200° C.
 23. The implantable article of claim 12 wherein theultrahigh molecular weight polyethylene blend is annealed afterirradiation at a temperature of about 50° C.